Well, first I ask my teacher! :) But if Teacher isn't available at the moment, I do what was stated above: pick apart the problem area and go over it slowly (and over, and over and over and over and over and so on and so forth etc.), then try to put it in context playing the whole piece under tempo. With fiddle playing I have found (thanks to Teacher) that there are very subtle ways of adjusting hand position, fingers, etc., that can make a HUGE difference in being able to play or not play a particular passage. I'm sure it's the same with other instruments as well.

I do the same thing with singing, although I'm on my own there!